Field of the Invention
The present invention relates to inspection apparatus and methods usable, for example, to perform metrology in the manufacture of devices by lithographic techniques. The invention further relates to an illumination system for use in such inspection apparatus and to methods of manufacturing devices using lithographic techniques.
Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
In lithographic processes, it is desirable frequently to make measurements of the structures created, e.g., for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes, which are often used to measure critical dimension (CD), and specialized tools to measure overlay, the accuracy of alignment of two layers in a device. Recently, various forms of scatterometers have been developed for use in the lithographic field. These devices direct a beam of radiation onto a target and measure one or more properties of the scattered radiation—e.g., intensity at a single angle of reflection as a function of wavelength; intensity at one or more wavelengths as a function of reflected angle; or polarization as a function of reflected angle—to obtain a diffraction “spectrum” from which a property of interest of the target can be determined.
Examples of known scatterometers include angle-resolved scatterometers of the type described in US2006033921A1 and US2010201963A1. The targets used by such scatterometers are relatively large, e.g., 40 μm by 40 μm, gratings and the measurement beam generates a spot that is smaller than the grating (i.e., the grating is underfilled). In addition to measurement of feature shapes by reconstruction, diffraction based overlay can be measured using such apparatus, as described in published patent application US2006066855A1. Diffraction-based overlay metrology using dark-field imaging of the diffraction orders enables overlay measurements on smaller targets. Examples of dark field imaging metrology can be found in international patent applications US20100328655A1 and US2011069292A1 which documents are hereby incorporated by reference in their entirety. Further developments of the technique have been described in published patent publications US20110027704A, US20110043791A, US2011102753A1, US20120044470A, US20120123581A, US20130258310A, US20130271740A and WO2013178422A1. These targets can be smaller than the illumination spot and may be surrounded by product structures on a wafer. Multiple gratings can be measured in one image, using a composite grating target. The contents of all these applications are also incorporated herein by reference.
As is known, each product and process requires care in the design of metrology targets and the selection of an appropriate metrology ‘recipe’ by which overlay measurements will be performed. In the known metrology technique, diffraction patterns and/or dark field images of a metrology target are captured while the target is illuminated under desired illumination conditions. These illumination conditions are defined in the metrology recipe by various illumination parameters such as the wavelength of the radiation, its angular intensity distribution (illumination profile) and its polarization. The inspection apparatus includes an illumination system comprising one or more radiation sources and an illumination system for the delivery of the illumination with the desired illumination parameters. In practice, it will be desired that the illumination system can switch between different modes of illumination by changing these parameters between measurements. In the following, the term ‘light’ will be used for convenience to refer the illuminating radiation, without implying any limitation to visible wavelengths.
In one commercially available apparatus, the illumination system includes firstly a pair of shutters and polarizing beam splitter device, to select between two possible polarization of radiation. This is followed by an aperture selection device, defining the desired angular distribution of the light. There are several moving parts in the known illumination system, making it sensitive to vibrations and wear. These moving parts are in both the selection of polarization and the aperture selection device. Another published patent application US20130141730A1 proposes to generate illumination profiles with customized color and/or polarization distribution. The customized profiles in that case are achieved by switching between different fibers of a fiber bundle. A fiber switching system is provided to couple a fiber with desired color and polarization of light to a fiber delivering light to a specific location in an illumination pupil.
As an alternative to moving aperture selection devices, some of the above publications mentioned that a programmable spatial light modulator (SLM) such as a deformable mirror array or liquid crystal (LC) transmissive SLM can be used also. In principle, such devices should enable a more compact design with fewer moving parts. However, in practice these devices have not been implemented for the illuminator. One reason for this may be that providing reflective programmable SLMs such as DMDs in the illumination path requires convoluted beam paths and cause layout difficulties. A problem with LC shutter type devices is that they generally deliver only one polarization of light, while metrology applications require freedom to control polarization as an illumination parameter in the recipe.
US20070279630 (KLA) discloses an ‘order selected’ microscope for overlay metrology in semiconductor manufacturing. It is said that a spatial light modulator (SLM) device can be provided in one or both of an illumination path and an imaging path. Examples of SLM include chrome-on-glass patterns, and may provide an apodization function, rather than selection. For the imaging path, it is mentioned that a liquid crystal transmissive or reflective pixellated element or a DMD (digital micro-mirror device) may be used. However, this is not mentioned for the illumination path, and polarization as an illumination parameter is not discussed at all.